Method of etching silicon

ABSTRACT

Silicon ( 12 ) is etched through a mask ( 11 ) comprising a layer of organic resin material (such as novolac) through which openings ( 32 ) are formed in the areas to be etched. The layer of organic resin is first deposited over a free surface of the device to be etched. The openings ( 32 ) are then formed by depositing droplets of a caustic etchant such as sodium hydroxide (NaOH) or potassium hydroxide (KOH) with an inkjet printer. The etchant reacts with the resin to expose the silicon surface in areas to be etched. The etching of the silicon surface is performed by applying a dilute solution of hydrofluoric acid (HF) and potassium permanganate (KMnO 4 ) to the exposed surface through the openings in the mask to etch the silicon to a desired depth ( 83 ).

FIELD OF THE INVENTION

The present invention relates generally to the field of semiconductordevice fabrication and in particular the in invention provides animproved processing step for use in a method of forming metal contactsand other structures in thin film semiconductor devices. A new devicestructure for thin film photovoltaic devices is also provided.

BACKGROUND OF THE INVENTION

A major advantage of thin-film photovoltaic (PV) modules overconventional wafer-based modules is the potential for low cost ofproduction. However in practice cost savings have been difficult toachieve as a major component of cost is the number and complexity ofprocess steps involved in the manufacturing sequence and can quicklyoutweigh savings in material costs. In particular the number of stepsthat require precise alignment, or the speed of the equipment used toperform a step can have a strong bearing on cost, as can the robustnessof a process, which might in some cases lead to additional remedialsteps being required or result in lower performance of the end productbecause of materiel degradation. Therefore, process improvements whichreduce alignment requirement, reduce the number of steps, reduce damageto the device or, allow a step to be performed more quickly providesignificant advantages.

SUMMARY OF THE INVENTION

The present invention provides a method of etching silicon through amask comprising the steps of:

-   -   a) Forming a layer of organic resin as a mask over a free        surface of the device to be etched;    -   b) Forming openings in the mask to expose the silicon surface in        areas to be etched;    -   c) Applying a dilute solution of hydrofluoric acid (HF) and        potassium permanganate (KMnO₄) to the silicon surface exposed        through the mask to thereby etch the silicon to a desired depth.

Preferably the area of silicon to be etched has a width and length whichare significantly greater (say by at least an order of magnitude) thanthe depth to be etched. In preferred embodiments the silicon to beetched is a thin film of silicon (such as polycrystalline silicon) on aforeign substrate and the etch is limited by the silicon being etchedsubstantially down to) the substrate. However the process is equallyapplicable to single crystal material (ie wafer material) and can bemade to progress at a rate which allows depth of etch to be controlledby timing of the etch. The depth can be controlled to remove only a thinsurface layer (eg to provide a clean surface for further processing) orcan remove a significant thickness of the silicon material)

Preferably the dilute solution of HF and KMnO₄ comprises a solution of1% HF and 0.1% KMnO₄. With this solution 1.5 μm of silicon willsubstantially etch away in 12 minutes at room temperature (21° C.).

The organic resin is preferably novolac, but other similar resins arealso suitable such as commonly available photoresists. The openings inthe resin layer can be formed by chemical removal using solutions ofcaustic substances such as potassium hydroxide (KOH) or sodium hydroxide(NaOH). In a preferred method according to the invention, droplets ofdilute (15%) potassium hydroxide are dispensed at locations intended foretching. The KOH solution is preferably deposited using ink-jet printtechnology. Other methods of making openings in the mask layer includelaser ablation and photographic techniques (using photoresist).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings (not drawn to scale) inwhich:

FIG. 1 is a diagram of a section through a semiconductor device afterinitial steps of applying an anti-reflection coating over a glasssubstrate and depositing a doped semiconductor film over theanti-reflection coating;

FIG. 2 is the sectional view seen in FIG. 1 after a scribing step hasbeen completed to form a cell separating groove dividing separate cellareas and insulating layers have been applied over the semiconductorlayer;

FIG. 3 is a schematic diagram of an X-Y table with an inkjet print headfitted for directly applying the insulation etchant, using inkjettechnology;

FIG. 4 is the sectional view seen in FIG. 2 (shifted slightly to theleft), after a pattern of etchant has been directly deposited onto theinsulating layer to open the insulating layer in areas where contacts toan underlying n⁺ type region of the semiconductor layer are required;

FIG. 5 is the sectional view seen in FIG. 4 after the insulation layerhas been opened in the areas where contacts to the underlying n⁺ typeregion of the semiconductor layer are required;

FIG. 6 is the sectional view seen in FIG. 5 after further etching stepshave been performed to remove some of the doped semiconductor film inthe area where the contact to the underlying n⁺ type region of thesemiconductor layer is required;

FIG. 7 is the sectional view seen in FIG. 6 after a reflow step to flowsome of the insulating layer into the hole formed by removal of some ofthe doped semiconductor film in the area where a contact to theunderlying n⁺ type region of the semiconductor layer are required. Apattern of caustic solution has been directly deposited onto theinsulating layer to open the insulating layer in an area where a contactto an upper p⁺ type region of the semiconductor layer is required;

FIG. 8 is the sectional view seen in FIG. 7 after the caustic has openedthe insulation layer in the areas where the contact to the upper p⁺ typeregion of the semiconductor layer is required;

FIG. 9 is the sectional view seen in FIG. 8 after further etching stepshave been performed to clean the surface of the doped semiconductor filmof damaged material in the areas where the contact to the upper p⁺ typeregion of the semiconductor layer is required;

FIG. 10 is the sectional view seen in FIG. 9 after a metal layer hasbeen applied to contact the p⁺ and n⁺ type regions of the semiconductormaterial and to interconnect adjacent cells;

FIG. 11 is the sectional view seen in FIG. 10 after the metal layer hasbeen interrupted to separate the contacts to the p⁺ & n⁺ type regionsfrom each other within each cell;

FIG. 12 is a back view (silicon side) of part of the device of FIG. 11;and

FIG. 13 is a diagram of a part of a completed device, illustrating theinterconnection between adjacent cells.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, FIG. 1 illustrates a part of a semiconductorstructure 11 which is a precursor to the photovoltaic device fabricationprocess described below. The semiconductor structure 11 is formed as athin semiconductor film applied to a substrate 22 in the form of a glasssheet to which a thin silicon nitride anti-reflection coating 71 hasbeen applied. The anti-reflection coating 71 has a thickness of 80 nm.For optimal performance, the thin semiconductor film comprises a thinpolycrystalline silicon film 12 formed with a total thickness in therange of 1 to 2 μm and preferably 1.6 μm. The polycrystalline siliconfilm 12 has an upper p⁺ type region 13 which is 60 nm thick, a lower n⁺type region 15 which is 40 nm thick, and a 1.5 μm thick intrinsic orlightly p type doped region 14 separating the p⁺ and n⁺ type regions.The sheet resistance in both n⁺ type and p⁺ type layers is preferablybetween 400 and 2500Ω/□, with no more than 2×10¹⁴ cm⁻² boron in total.Typical values are around 750Ω/□ for n⁺ type material and 1500Ω/□ for p⁺type material. The thickness of the n⁺ type and p⁺ type layers istypically between 20 and 100 nm. The glass surface is preferablytextured to promote light tapping, but this is not shown in the drawingsfor sake of clarity.

Division into Cells

As seen in FIG. 2, the silicon film 12 is separated into cells byscribed isolation grooves 16. This is achieved by canning a laser overthe substrate in areas where isolation grooves 16 are required to definethe boundaries of each photovoltaic cell. To scribe the grooves 16, thestructure 11 is transferred to an X-Y stage (not shown) located under alaser operating at 1064 nm to produce focussed laser beam 73 which cutsthe isolation grooves through the silicon. The laser beam is focussed tominimise the width of the groove, which is lost active area. Typically,a pulse energy of 0.11 mJ is required to fully ablate the silicon filmand gives a groove width of 50 μm. To ensure a continuous groove,successive pulses are overlapped by 50%. The optimum cell width is inthe range of 5 to 8 mm and cell widths of 6 mm are typical.

As seen in FIG. 2, two layers of insulation are preferably used on thesurface of the silicon and are added after the laser scribing stepdescribed above. The first insulation layer is an optional thin buttough cap nitride 72. This layer protects the exposed silicon along theedges of the cell definition grooves 16 after laser scribing andpassivates the surface of the silicon. The cap nitride 72 is preferablycapable of being etched completely in a few minutes to allow access tothe silicon at n type and p type contact locations and typicallycomprises 60 nm of silicon nitride deposited by PECVD at a temperatureof 300-320° C.

Before the cap layer 72 is applied, the structure 11 is transferred to atank containing a 5% solution of hydrofluoric acid for one minute. Thisremoves any remaining debris and any surface oxides that may haveformed. The structure is rinsed in de-ionised water and dried.

The second insulation layer 17 is a thin layer of organic resin. Theinsulating resin is resistant to dilute solutions of hydrofluoric acid(HF) and potassium permanganate (KMnO₄), and is preferably vacuumcompatible to 10⁻⁶ mbar. The insulation material most often used isnovolac resin (AZ P150) similar to that used in photoresist (but withoutany photoactive compounds). The novolac resin is preferably loaded with20-30% white titania pigment (titanium dioxide) which improves coverageand gives it a white colour that improves its optical reflectivity tohelp trap light within the silicon. The resin layer 17 serves as an etchmask for etching steps described below and also covers over the roughjagged surface that is formed along the edges of the cell definitiongrooves 16, an area that is prone to pinholes in the cap nitride layer72. The organic resin layer 17 also thermally and optically isolates themetal layer from the silicon to facilitate laser patterning of a metallayer in contact forming process steps described below.

The novolac resin is applied to each module to a thickness of 4 to 5 μmusing a spray coater. After the structure 11 is coated, it is passedunder heat lamps to heat it to 90° C. to cure. As seen in FIG. 2, theinsulation layer 17 is applied over the cap layer 72 and extends intothe cell separation grooves 16.

Opening Mask and Etching n Type Contact Openings

In order to make electrical contact to the buried n⁺ type layer and theupper p⁺ type layer with a metal layer which will be subsequentlyformed, holes must be made through the novolac resin layer 17 and thecap nitride layer 72 in the locations where the n type “crater” contactsand the p type “dimple” contacts are required. Firstly with regard tothe “crater” contacts to the buried n⁺ type silicon layer, as wellopening the novolac resin layer 17 and the cap nitride layer 72, most ofthe silicon film 12 must be removed from areas beneath what will laterbecome the n type metal pads to form the n type contact openings 32.Referring to FIGS. 3, 4 and 5 ink-jet technology is used to open holesin the novolac resin layer 17 at the crater locations. To achieve thisthe structure 11 is loaded onto an X-Y stage equipped with an ink-jethead 91 having multiple nozzles with a nozzle spacing of 0.5 mm andcontrolled by controller 92. The glass is held down with a vacuum chuckand initially scanned to ensure that no point is deformed more than 1 mmabove the stage. The glass is then scanned beneath the head 91 at atable speed of typically 400 mm/s. Droplets 76 of dilute (15%) potassiumhydroxide (KOH) (see FIG. 4) are dispensed at locations intended for ntype ‘crater’ contacts. The odd-numbered nozzles fire in theodd-numbered cells, and the even-numbered nozzles fire in theeven-numbered cells, so that within a given cell, the spacing betweenlines of droplets is 1 mm. The spacing between droplets within each lineis 400 μm, hence the rate of droplet release at a table speed of 400mm/s is 1 kHz. The droplets are sized to etch circular openings in theresin layer that are about 100 μm in diameter. The KOH solution removesthe resin insulation 17 in the area of the droplet 76 after a fewminutes to form the hole 32 seen in FIG. 5.

The openings 32 are spaced holes so that lateral continuity ismaintained in the semiconductor layer after contact formation. Theink-jet printing process applies a droplet 76 of the caustic solution ina controlled manner to remove the insulation only where the n typecontacts are to be formed. The caustic solution preferably containspotassium hydroxide (KOH) but can also use sodium hydroxide (NaOH) andincludes glycerol for viscosity control. The print head used for thispurpose is a model 128ID, 64ID2 or 64-30 manufactured by Ink JetTechnologies Inc., and will print substances having a viscosity in therange 5 to 20 centipoise. The droplet size deposited by the print headis in the range of 20 to 240 picolitre corresponding to a depositeddroplet diameter range of 50-150 μm. In the preferred embodiment thedroplets are printed at a diameter of 100 μm. It should be noted thatnovolac is an organic resin closely related to the resins used inphoto-resist material and the etchant printing process described abovewill apply equally to the patterning of other such materials.

To extend the opening 32 into the silicon layer 12 as seen in FIG. 6,the structure 11 is rinsed in water to remove residual KOH from theink-jet printing process, and it is then immersed in a tank containing a5% solution of hydrofluoric acid for 1 minute to remove the siliconnitride from the n type contact openings 32. The sheet is then directlytransferred toga tank containing 1% hydrofluoric acid (HF) and 0.1%potassium permanganate (KMnO₄) for 4 minutes. This time is long enoughto remove all of the p⁺ type layer and etch down along grain boundariesto expose some of the n⁺ type layer for the silicon thicknesses statedabove however the time should be adjusted for different silicon layerthicknesses, silicon crystal quality and extent of surface texturing.The structure 11 is then rinsed in de-ionised water and dried.

The resulting opening 32 in the silicon 12 has a rough bottom surface82, in which some points may be etched through to the anti-reflectionlayer 71 and some ridges 83 extend into the lightly doped p type region14 as seen in FIG. 6. However as long as some of the n⁺ type region isexposed, good contact can be made to the n⁺ type region. Because the ptype region is very lightly doped in the area near the n⁺ type regionthere is insufficient lateral conductivity to cause shorting if some ptype material is also left in the bottom of the hole 32.

Reflow of Mask

Because the side walls of the hole 32 are passing trough the p⁺ typeregion 13 and the lightly doped region 14, the walls need to beinsulated to prevent shorting of the p-n junction. This is achieved bycausing the insulation layer 17 to re-flow whereby a portion of theinsulation layer 78 in the vicinity of the edge of the opening 32 flowsinto the hole to form a covering 79 over the walls as seen in FIG. 7. Toachieve this the sheet is passed through a zone containing a vapour of asuitable solvent. This causes the novolac resin of the insulating layer17 to reflow, shrinking the size of the crater openings. 32. As thesamples exit this zone, they are heated under heat lamps to atemperature of 90° C. to drive out the remaining solvent.

The rate of re-flow will vary with the aggressiveness of the solventused, the concentration and, temperature. There are many suitable,volatile solvents that will dissolve organic resins such as novolac,including substances such as acetone. Acetone is a suitable solvent forthe process, but acts quite aggressively, requiring only a few secondsto cover the walls of the hole 32 with resin, and making it difficult tocontrol the pus accurately. The preferred solvent is propylene glycolmonomethyl ether acetate (PGMEA) and the device is introduced into anatmosphere containing a saturated vapour of PGMEA at room temperature(eg, 21° C.) for 4 minutes until a slight shrinkage of the holes in theinsulation is observed.

Opening Mask and Cleaning p Type Contact Openings

A further set of holes 19 (see FIG. 8) are then formed in the insulationlayer 17, again using the printing and etching process described abovewith reference to FIGS. 3, 4 and 5. These-openings are formed byprinting droplets 81 of caustic solution onto the insulation (see FIG.7) in the locations where p type contact “dimples” are required.Following the removal of the insulation layer 17 by the caustic solutionto form the openings 19 (see FIG. 8), any residual caustic solution iswashed off with water and the cap layer 72 removed in the openings 19with an etch of 5% hydrofluoric acid (HP) for 1 minute (note times offrom 10 seconds to 10 minutes may be required to remove the nitridelayer depending on its stoichiometry). Optionally, any damaged siliconmaterial on the surface of the p⁺ type region 13 is then removed toallow good contact using an etch in 1% hydrofluoric acid (HF) and 0.1%potassium permanganate (KMnO₄) for ten seconds followed by a rinse inde-ionised water to provide the slightly recessed contact “dimple” 85seen in FIG. 9. This length of etch is long enough to remove surfaceplasma damage without etching all the way through the p⁺ type layer 13.It is also short enough to have negligible impact on the n typecontacts.

Formation of Metal Contacts

The final stage of device fabrication involves depositing a metal layerand slicing it up so that it forms a plurality of independent electricalconnections, each one collecting current from one line of p type dimplecontacts and delivering it to a line of n type crater contacts in theadjacent cell. In this manner, monolithic series interconnection of thecells is achieved.

Before the metal layer is applied, the structure 11 is immersed into atank containing a 0.2% solution of hydrofluoric acid for 20 seconds.This acid removes the surface oxide from both the crater and dimplecontacts. There is wide latitude for the strength and duration of thisetch. The structure is then rinsed in deionised water and dried.

Turning to FIG. 10, the contact metal for the n type and p type contactsis applied simultaneously by depositing a thin metal layer 28 over theinsulation layer 17 and extending into the holes 32 and 19 to contactthe surfaces 82 and 85 of the n⁺ type region 15 and p⁺ type region 13.The metal layer is preferably a thin layer of pure aluminium, whichmakes good electrical contact to both n⁺ type and p⁺ type silicon,provides good lateral conductivity, and has high optical reflectance.The aluminium thickness is typically 100 nm.

Isolation of n an p Type Contacts

The isolation of the n type end p type contacts is achieved by using alaser 86 (see FIG. 10) to melt and/or evaporate the metal layer 28 tothereby form an isolation groove 31 as seen in FIG. 11. When the laseris pulsed on, a small amount of metal is ablated directly under the beamcreating a hole 31.

The structure 11 is processed using a laser operating at 1064 nm toscribe the isolation grooves in the metal layer 28. The laser isadjusted so that it scribes through the metal layer 28 without damagingthe silicon 12. These scribes 31 separate the n type contacts 32 fromthe p type contacts 19 within each cell, while retaining the seriesconnection of each cell to its neighbours. Preferred laser conditionsare a pulse energy of 0.12 mJ with the beam defocused to a diameter ofabout 100 μm. The pulse overlap is 50% and the scribes are spaced 0.5 mmapart. In addition, there are discontinuous scribes 34 along each celldefinition groove 16 (see FIG. 12).

FIG. 12 illustrates a rear view of a part of a device made by theprocess described above, from which it can be seen that each of thecells of the device 11 comprises an elongate photovoltaic element 35 a,35 b, 35 c, 35 d divided across its long axis by a plurality oftransverse metal isolation scribes 31 which isolate alternate sets ofholes 19 and holes 32 respectively providing contacts to the p⁺ type andn⁺ type regions of the cell. The transverse scribes 31 are made as longsubstantially straight scribes extending over the length of the devicesuch that each scribe crosses each elongate cell.

Following the formation of the first set of scribes 31, a fewer set ofmetal isolation scribes 34 are formed over the cell separation scribes16 between adjacent cells 11, to isolate every second pair of cells. Themetal isolation scribes 34 extending to either side of any one of theelongate transverse scribes 31 are offset by one cell with respect tothose on the other side of the same transverse scribe 31 such that thecells become series connected by a matrix of connection links 36 withalternating offsets, connecting one set of p type contacts 19 of onecell 35 to a set of n type contacts 32 of an adjacent cell 35, as shownin FIG. 12.

The metal isolation scribes 31 comprises a first set of long scribestransverse to the cells 35 from 50-200 μm wide, preferably about 100 μmwide. The scribes are typically spaced on centres of 0.2-2.0 mm andpreferably about 0.5 mm to form conducting strips about 0.2-1.9 mm andpreferably about 0.4 mm wide. The isolation scribes 34 comprises asecond set of interrupted scribes parallel to the long direction of thecells 35 and substantially coincident with the cell isolation grooves 16in the silicon, The isolation scribes 34 are also from 50-200 μm wide,preferably about 100 μm wide. It is equally possible to form theisolation scribes 34 before forming the transverse isolation scribes 31.The scribed areas are illustrated in FIG. 12 with cross-hatching.

A portion of the completed-structure is illustrated in FIG. 13 whichshows the connection of an n type contact of one cell to the p typecontact of an adjacent cell to provide a series connections of cells. Inpractice there may be several n type contacts grouped together andseveral p type contacts grouped together however for the sake of clarityonly one of each is shown in each cell. The arrangement shown in FIG. 13is also schematic as the isolation grooves 16 in the silicon and theisolation grooves 31 in the metal run perpendicularly to one mother inpractice as is seen in FIG. 12.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

1. A method of etching silicon through a mask comprising: (a) forming alayer of organic resin as a mask over a free surface of a device to beetched; (b) placing the device on a stage; (c) locating an ink-jet printdevice over the device to be etched and in close proximity thereto, theink-jet device and stage being moveable relative to one another; (d)using an additive to adjust the viscosity of a reactive material to thatrequired by the ink-jet device; (e) supplying the ink-jet device withthe reactive material with the additive added; (f) moving the device andthe ink-jet device relative to one another under control of controlmeans; (g) controlling the ink-jet device to deposit predeterminedamounts of the reactive material onto a surface of the mask in apredetermined pattern as the device and the ink-jet device move relativeto one another, to form openings in the mask, thereby exposing thesilicon surface in areas to be etched; and (h) applying a dilutesolution of hydrofluoric acid (HF) and potassium permnanganate (KMnO₄)to the silicon surface exposed through the mask to thereby etch thesilicon to a desired depth.
 2. The method of claim 1, wherein thematerial to be etched is a thin film of silicon on a foreign substrate.3. The method of claim 1, wherein the area of silicon to be etched has awidth and length which are at least an order of magnitude greater thanthe depth to be etched.
 4. The method of claim 1, wherein the silicon tobe etched is crystalline silicon.
 5. The method as claimed in claim 1,wherein the organic resin material is novolac.
 6. The method of claim 1,wherein the stage is an X-Y stage and the ink-jet device is fixed, suchthat relative motion of the device to be etched and the ink-jet deviceis achieved by moving the stage under the ink-jet device.
 7. The methodas claimed in claim 1, wherein the openings in the mask are formed usinga solution of potassium hydroxide (KOH) or sodium hydroxide (NaOH). 8.The method of claim 1, wherein the viscosity of the reactive material isadjusted to be in the range of 5 to 20 centipoise.
 9. The method ofclaim 8, wherein the viscosity adjusting additive is glycerol.
 10. Themethod claim 1, wherein the etch is performed until at least a portionof a substrate is exposed in each area to be etched.
 11. The method ofclaim 10, wherein the etch substantially completely removes the siliconfrom the substrate in each area to be etched.
 12. The method of claim10, wherein the silicon to be etched is a thin film of polycrystallinesilicon on a foreign substrate.